you cant get back a regular pet if you dismiss it unless you want to catch a new one and train it back up to what ever lvl you dismissed the old one at, but if you dismiss a human pet all you have to do is go back to where you did the last part of the pet quest at.
Aries:
Fire rb 140+ killer croiu
Water rb 130+ wit Scarr
Earth rb 130+ knight Phobose
Fire rb 120+ wit Soneko